32 



L. Leigh Fermor — Laterites of French Guinea. 



wrong in attempting to restrict the term to the colloidal form of 

 kaolinite. The term lithomarge has long been used in the literature 

 of Indian geology for the variegated clays so often underlying the 

 Indian laterites, x and has by several decades the priority over 

 halloysite in application to these clays, and it seems therefore 

 desirable to me to retain it in this connexion. Perhaps in the same 

 way as bauxite has been shown to be a rock rather than a mineral, 

 lithomarge must be so regarded. In fact, we might regard lithomarge 

 as related to kaolinite and halloysite in the same way as bauxite is 

 related to the minerals gibbsite and the alumogels (see infra). 



With this definition of the term lithomarge we may translate and 

 re-arrange Professor Lacroix' improved classification as follows : — 



After closing his introduction with a short historical resume of the 

 previous work on the laterites of Guinea, 4 Professor Lacroix deals in 

 four chapters with the following subjects : (i) The products of 



1 Thus in the publications of the Geological Survey of India it is used by 

 W. T. Blanford as early as 1859 ; see " Note on the Laterite of Orissa " (Mem. 

 Geol. Surv. Ind., i, p. 286). Blanford clearly distinguishes between the overlying 

 laterite and the underlying lithomarge produced by the decomposition of gneiss. 

 He writes : ' ' The underlying form varies so much in constitution according to 

 the rock from which it is derived, that scarcely any petrol ogical term will 

 include all its varieties. As a rule, however, it is, when derived from gneiss or 

 other felspathic rock, a more or less ferruginous clay varying in purity. To 

 such substances the name Lithomarge has frequently been applied, and it 

 seems the most applicable in the present instance, it being understood that all 

 the impure varieties derived from quartzose metamorphic rocks or sandstone 

 are here included in the term." He notes the "apparent passage of 

 Lithomarge into Laterite ' ' , but gives a different explanation from that of 

 Professor Lacroix. 



2 By using the term lithomargic where Lacroix uses argileuse, we have the 

 adjective argillaceous or argileuse, available as a comprehensive term, including 

 both ' kaolinic ' and ' lithomargic ' . 



3 I prefer the term lateritic constituents (L.C.) to hydrates, because, at 

 the surface, laterite sometimes contains the anhydrous mineral hematite, 

 developed as a result of dehydration. 



4 In which, by the way, the author overlooks Mr. J. Morrow Campbell's 

 paper entitled "The Origin of Laterite" (Trans. Inst. Min. Met., xix, 

 pp. 432-57, 1910), based largely on observations made in Haute Guinee. 



